1. Technical Field
The present invention relates generally to foldable dish reflectors and, more particularly, to implementation of reflective surfaces for foldable dish reflectors that are suitable for higher radio frequencies and solar energy concentration
2. Description of the Related Art
Foldable dish reflectors are commonly used for radio antennas and solar collectors in terrestrial and space based applications. One conventional approach to implementation of systems of this type makes use of a foldable framework that can support a reflective surface. A wide variety of structures have been developed for such foldable framework systems. Reflective surfaces are conventionally mounted to these structural supports.
Conventional deployable reflectors have typically made use of one two basic types designs for reflector surfaces. One approach uses a segmented solid reflector surface made from rigid or semi-rigid panels arranged on a supporting structure that can be folded within a spacecraft prior to launch. A second design is comprised of a mesh material arranged on a support structure.
The segmented solid surface approach is shown in U.S. Pat. No. 5,104,211. The structure approximates compound curvature surfaces by using a three-dimensional arrangement of compactly stowable flat reflective panel segments. The semi-rigid panel segments are deployed on an umbrella-like framework of radially extending ribs, struts and cords. The ribs, struts and cords deploy away from a central hub to form a system of radial trusses.
Similarly, U.S. Pat. No. 6,229,501 also illustrates a segmented solid surface approach. The system uses a number of individual hexagonal reflectors that can be arranged around a rigid central element. The reflectors are made from foldable, form stable CFK Carbon-Faser-verstxc3xa4rkter Kunststoff (German synonym for Carbon-Fiber-Reinforced Plastic xe2x80x9cCFRPxe2x80x9d) that has been coated with a metalized foil. The respective reflectors are folded or deployed in the manner of an umbrella.
Yet another solid surface design is disclosed in U.S. Pat. No. 4,860,023 wherein a parabolic reflector antenna for telecommunication satellites implements a reflector using a honeycomb core sandwiched between two Kevlar sheets. A metal grid is applied to the surface of at least one of the Kevlar sheets for establishing a surface sensitive to the frequency of the RF signals.
U.S. Pat. No. 5,198,832 discloses a mesh type reflective surface that has been used for deployable reflector systems. The system uses flexible polyester knitted mesh fabric to form the reflector surface. The fabric is plated with a reflective metal coating and is designed to be elastic, particularly in a radial direction.
U.S. Pat. No. 6,313,811 also discloses a deployable antenna that utilizes a mesh type reflective surface. The system uses includes radial and hoop support members for deploying a surface, such as a metallic mesh antenna material that is reflective of electromagnetic energy. Similarly, U.S. Pat. No. 6,278,416 discloses a system of cords and ties for supporting a metallic mesh reflector surface.
One problem with solid panel type reflector surfaces is the inherent complexity of folding rigid segmented panels. Another problem such rigid panel systems are the weight and volume associated with their deployment. Further, in their deployed configuration, segmented rigid or semi-rigid panels generally have a small gap or overlap between adjacent panels. Discontinuous areas such as these can be detrimental to Radio Frequency (RF) performance because they cause products of intermodulation (PIM). Additionally these discontinuities can disperse the reflected RF energy in undesirable directions that create or increase RF sidelobes.
The mesh approach solves many of these problems as it facilitates inherently simpler deployment and lighter weight. However, mesh materials are not suitable for all reflector applications. In conventional systems, the reflector material has been formed of a metallic or metal plated mesh material. When tensioned by the support structure, the conventional mesh material will define interstices or spaces between the fibers or filaments forming the mesh. These interstices limit the usefulness of currently available mesh material as reflector surfaces, particularly for frequencies above about 15 GHz.
It is possible that tighter mesh designs will eventually facilitate operation at frequencies ranging from 20 to 30 GHz. Beyond these frequencies, however, mesh solutions to the reflector problem exhibit increased loss and therefore become impractical. Further, mesh designs are simply not suitable for use in other applications such as solar concentrators.
In addition to the mesh reflectivity loss due to interstices, there are also electrical conductivity effects. To explain, one must understand the basics of knit mesh. Mesh is knit on machines that feeds-in individual gold plated wires, performs the knitting operation, and outputs knitted mesh. Thus in one direction, the direction of knit, the mesh inherently should have excellent electrical conductivity, as the wires are continuous in this knit direction. However for the mesh to maintain electrical conductivity in the direction perpendicular to this knit direction, the mesh must be tensioned sufficiently to ensure adequate contact pressure between individual wire elements. This requires that the mesh be tensioned in this lateral direction. The lateral tension, due to the material behavior of the mesh, generates a tension in the knit direction as well. Thus in order to maintain electrical conductivity to achieve the necessary RF reflectivity, the mesh material must be tensioned in both the knit and lateral directions.
Another reason the mesh must be tensioned is geometric. Mesh in its untensioned state, does not maintain a smooth, semi-flat shape. It must be tensioned in order to impose and hold a reasonably flat surface. Depending upon the characteristics of the particular mesh material and knit, the amount of tension required to maintain an adequately smooth, semi-flat shape can vary. The tension must also be adequate to smooth-out any wrinkles or other imperfections that may be present as the mesh is pulled into its deployed shape from its stowed condition. If the deployed, tensioned mesh is insufficiently smooth, the geometric effects may lead to additional RF loss due to surface roughness.
This presence of tension in the mesh required, as explained above, to meet surface roughness and electrical conductivity requirements has another detrimental effect. There are two components to this effect. The first relates to the flat facet approximation to the parabolic surface generally employed in reflector antenna. Assuming for a moment that the tensions in the mesh are adequate to ensure geometric flatness and electrical reflectivity, one can further assume that the mesh forms a flat facet between each set of tie points that are held by the cord/tie or other backup structure. The desired parabolic surface reflector is a doubly curved surface. Assuming the mesh at the tie points is held correctly to coincide with the parabolic surface then, between these points, the mesh will necessarily deviate from the desired, doubly curved surface even if the mesh between the tie points lies in a perfectly flat plane. This is the flat facet approximation. The degree of deviation from the desired parabolic surface can be improved to some extent by making the distance between tie points smaller.
The flat facet approximation is the first part of the detrimental tension effect. The second has to do with the true behavior of a pre-tensioned membrane. The flat facet approximation assumes the mesh between tie points is ideally flat. However, the physical behavior of tensioned mesh shows that the mesh, between tie points, is not flat. Instead, the mesh bulges up above the flat facet, towards the focal point of the paraboloid. This can be accurately predicted through use of the equations governing a doubly curved, pre-tensioned membrane. It has also been validated by experimental measurement and on numerous deployable reflector surfaces. The bulge is dependent upon the focal length of the paraboloid and the tension in the membrane.
Thus, the geometric surface accuracy achievable with tensioned mesh is limited by the tension itself. Even if the density of the mesh is increased to provide adequate RF reflectivity at higher frequencies, the geometric limit due to these detrimental tension effects is another limiting factor to overcome.
The ideal surface is one that has a high degree of flexibility and folds readily like mesh to stow, yet needs no pre-tension to maintain its deployed geometry. Accordingly, there is a need for a suitable reflector surface system that can provide performance at higher radio frequencies while avoiding the deployment problems associated with rigid or semi-rigid solid surface reflectors.
The invention concerns a deployable solid surface reflector. The deployable reflector includes a support structure that is formed from a group of support members. The support members are movable from a compact stowed configuration to a deployed configuration such that selected portions of the support members define a prescribed surface when in the deployed configuration. For example, the prescribed surface can be a parabolic surface. A continuous reflector material formed from a flexible solid surface is provided and restrained against the support members defining the prescribed surface.
According to one aspect of the invention, the support structure can be comprised of a plurality of radially extending support ribs and a plurality of circumferentially extending support cords. The support cords can define the prescribed surface between adjacent ones of the support ribs.
In order to be movable from a stowed configuration to a deployed configuration, the continuous reflector material can have a bending stiffness of between about 0.1 to 10 inch-pounds. Further, for space-based applications, the continuous reflector material preferably has a relatively low coefficient of thermal expansion of less than about 1.0xc3x9710xe2x88x926/xc2x0 F. or one part per million (ppm) per degree Fahrenheit.
According to one aspect of the invention, the deployable reflector material can be comprised of a laminate comprising a woven quartz cloth material or a unidirectional quartz lamina, each pre-impregnated with a resin such as an epoxy resin. Alternatively, the material can be a laminate comprising graphite and epoxy. If the material used to form the solid surface is non-conductive or non-reflective, then a suitable reflective layer such as aluminum or some other metal layer can be applied to provide the reflective properties.